An analysis of turbulence in the dye medium of a high-repetition-rate narrowband dye laser is presented, and it is shown that the output bandwidth is proportional to the turbulence length scale, which depends on the Reynolds number. The length scale and the bandwidth both first decrease and then increase as the Reynolds number is increased from -200 to -20,000. The observed bandwidth, as we report and as in the published literature, confirms the validity of the analysis. A Rhodamine 6G grazing-incidence grating, stable dye laser transversely pumped by a copper-vapor laser was used in the experiments.

Gain medium inhomogeneity is inevitable in a high repetition rate dye laser. In this paper, gain medium inhomogeneity coupled with a high repetition rate dye laser and their influences on the spectral fluctuations are investigated. The wavelength fluctuates within ±0.025 nm while the bandwidth fluctuates within ±1.108 GHz by reducing the flow Reynolds number from 1012 to 221 in the laminar region of flow. The wavelength fluctuations increase from 0.0110 to 0.0200 nm while the bandwidth fluctuations increase from 0.296 to 0.509 GHz by increasing the Reynolds number from 5645 to 6774 in the turbulent region. The heat deposition as a function of distance from the window wall in the gain medium and heat transfer coefficients as a function of Reynolds numbers in laminar and turbulent flow were estimated. Enlargement of the turbulent attribute (through a laminar sub-layer and buffer region) in the gain medium as a function of the Reynolds number was used to illustrate the observed spectral fluctuation trends of the dye laser.

We present investigations of the presence of oxides, hydrides, and hydrocarbons in high-purity (residual resistivity ratio, ∼300) niobium (Nb) materials used in fabrication of superconducting radio frequency (SRF) cavities for particle accelerators. Raman spectroscopy of Nb materials (as-received from the vendor as well as after surface chemical- and thermal processing) revealed numerous peaks, which evidently show the presence of oxides (550 cm−1), hydrides (1277 and 1385 cm−1: ∼80 K temperature), and groups of hydrocarbons (1096, 2330, 2710, 2830, 2868, and 3080 cm−1). The present work provides direct spectroscopic evidence of hydrides in the electropolished Nb materials typically used in SRF cavities. Raman spectroscopy thus can provide vital information about the near-surface chemical species in niobium materials and will help in identifying the cause for the performance degradation of SRF cavities. Furthermore, photoelectron spectroscopy was performed on the Nb samples to complement the Raman spectroscopy study. This study reveals the presence of C and O in the Nb samples. Core level spectra of Nb (doublet 3d5/2 and 3d3/2) show peaks near 206.6 and 209.4 eV, which can be attributed to the Nb5+ oxidation state. The core level spectra of C 1 s of the samples are dominated by graphitic carbon (binding energy, 284.6 eV), while the spectra of O 1 s are asymmetrically peaked near binding energy of ∼529 eV, and that indicates the presence of metal-oxide Nb2O5. The valence-band spectra of the Nb samples are dominated by a broad peak similar to O 2p states, but after sputtering (for 10 min) a peak appears at ∼1 eV, which is a feature of the elemental Nb atom.

In this paper, wavelength and bandwidth fluctuations of a Rhodamine 6G dye laser, transversely pumped by CVL, through a flowing gain medium are investigated. The gain medium mass flow rates were varied in the range 1–6 l min−1 (LPM). Analysis of the wavelength and bandwidth fluctuations of the dye laser was carried out using a high resolution Fabry–Perot etalon based setup and composite images generated from spectral profiles. The wavelength varied within 0.0290, 0.0240, 0.0200, 0.0120, 0.0110 and 0.0200 nm while the bandwidth was within 1.340, 0.912, 0.679, 0.301, 0.296 and 0.509 GHz at flow rates of 1.0, 2.0, 3.0, 4.0, 5.0 and 6.0 LPM, respectively. The minimum fluctuations, which depend on the experimental conditions, were observed at ∼5 LPM. Spectral fluctuations at particular flow rates depend mainly on the microscopic fluctuations in the temperature and flow velocity inside the boundary layers.

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